High-strength fluting from NSSC pulp

ABSTRACT

There is provided a method of producing a containerboard, comprising the step of pressing a web formed from a pulp comprising NSSC pulp in an extended nip press, such as a shoe press, wherein the line load in the extended nip press is above 1200 kN/m. Further, there is provided a corrugated board comprising a liner and a fluting, wherein the fluting is formed from a pulp comprising NSSC pulp, the density of the fluting is above 725 kg/m3 and the geometric SCT index (ISO 9895) of the fluting is above 37 Nm/g.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 15/531,263, now allowed, which is a § 371 National State Applicationof PCT/EP2015/077190 filed Nov. 20, 2015, claiming priority to EP14194891.9 filed Nov. 26, 2014.

TECHNICAL FIELD

The invention relates to a method of producing a containerboard to beused as fluting.

BACKGROUND

Neutral Sulfite Semi-Chemical (NSSC) pulping is an old process that itis well known in the field of paper pulping and in use in many pulpmills around the world. One of the reasons for using NSSC pulping is thehigh yield.

In NSSC pulping, the cooking liquor comprises sulfite, such as Na₂SO₃ or(NH₄)₂SO₃ and a base, such as NaOH or Na₂CO₃. “Neutral” means that thepH of the NSSC cooking liquor is generally between 6 and 10. Normally,the cooking time is between 0.5 and 3 hours and the cooking temperatureis 160-185° C. The NSSC pulp comprises comparatively high amounts ofresidual lignin, such as 15-20%, which make the NSSC pulp stiff. TheNSSC pulping is “semi-chemical” in the sense that it comprisesmechanical treatment/grinding.

The NSSC pulp is for example used to produce containerboard that issubsequently corrugated to form the fluting of corrugated board.

Examples of mills using the NSSC pulping method are: Mondi Swiecie S.A.'s mill in Swiecie (PM 4), Poland; Savon Sellu Oy's (Powerflute's)mill in Koupio, Finland; Stora Enso Oyj's mill in Heinola, Finland(Heinola Fluting Mill); Packaging Corp. of America's mills in Filer Cityand Tomahawk, United States; Ilim Group's mill (PM1 and PM3) in Kojazma,Russia (Kotlas Mill); JSC Arkhangelsk Pulp & Paper's mill (PM2) inNovodvinsk, Russia; Rock-Tenn Co.'s mill in Stevenson, United States;International Paper's mills in Mansfield, Pine Hill and Valliant, UnitedStates; Georgia Pacific LLC's mills in Big Island, Cedar Springs andToledo, United States and Norampac Inc's mills in Cabano and Trenton,Canada.

SUMMARY

One way of increasing the strength of corrugated board is to increasethe compressive strength of the containerboard used to form the fluting(i.e. the corrugated medium) of the corrugated board.

In is an object of one aspect of the present disclosure to provide amethod of producing a containerboard of increased strength from pulpcomprising NSSC pulp.

In is an object of another aspect of the present disclosure to provide acorrugated board having a corrugated medium of increased strength,wherein pulp comprising NSSC pulp is used to form the containerboard ofthe corrugated medium.

It is often desirable to reduce the density (i.e. increase the bulk) ofpaperboard as lower density is associated with lower consumption offibers/raw material. The present inventor has however realized that oneway of increasing the compressive strength of the containerboard is toincrease its density. Further, the inventor has realized that thedensity may be increased by wet pressing. It is however difficult topress a web from NSSC pulp to higher densities because of the stiffnessof the NSSC fibers.

A shoe press may be used for dewatering a paper web. Many times the shoepress is used for dewatering the paper web without reducing the bulk toomuch. The design of a shoe press is such that the nip is longer than inother types of presses. Thereby the press pulse in the shoe press islonger. The longer press pulse means that sufficient dewatering may beobtained in a shoe press at a maximum nip pressure that is lower than inother types of presses. This reduction in maximum nip pressure hastraditionally been used to save the bulk of the paper web.

The present inventor's solution to the above-identified problem ofpressing a web from NSSC pulp to a high density is to use a shoe pressat a very high line load. When a shoe press with such a high line loadwas used on a paper web from NSSC pulp, a high-density containerboard ofhigh compressive strength was obtained.

Another benefit of the present invention is also that the increasedcompressive strength may be achieved at a maintained or even increasedmachine speed.

The present disclosure thus provides a method of producing acontainerboard, comprising the step of pressing a web formed from a pulpcomprising NSSC pulp in an extended nip press, such as a shoe press,wherein the line load in the extended nip press is above 1200 kN/m. Thecontainerboard is intended for fluting in corrugated board.

Further, the present disclosure provides a corrugated board comprising aliner and a fluting, wherein the fluting is formed from a pulpcomprising NSSC pulp, the density of the fluting is above 725 kg/m³ andthe geometric SCT index of the fluting is above 37 Nm/g.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described, by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic drawing of the wire section used in pilot trials.

FIG. 2 is a schematic drawing of the press section used in pilot trials.

FIGS. 3-11 relates to results obtained in pilot trials.

FIG. 3 shows nip pressure profiles in a shoe press at different lineloads and a tilt of 1.3.

FIG. 4 shows nip pressure profiles in a shoe press at different tiltsand a line load of 1400 kN/m. The nip pressure profile at a tilt of 1.3and a line load of 1500 kN/m is also shown.

FIG. 5 shows the densities obtained when a shoe press was used atdifferent tilts at a line load 1400 kN/m.

FIG. 6 shows the densities obtained when a shoe press was used atdifferent line loads (306-1500 kN/m, resulting in different total pressimpulses) at a constant tilt (1.3). It also shows the density obtainedat a line load of 1500 kN/m when steaming was added (point 3017).

FIG. 7 shows the geometric SCT index values obtained when a shoe presswas used at different line loads (306-1500 kN/m, resulting in differenttotal press impulses) at a constant tilt (1.3). It also shows thegeometric SCT index obtained at a line load of 1500 kN/m when steamingwas added (point 3017).

FIG. 8 shows the SCT index values in the cross direction (CD) obtainedwhen a shoe press was used at different line loads (306-1500 kN/m,resulting in different total press impulses) at a constant tilt (1.3).It also shows the geometric SCT index obtained at a line load of 1500kN/m when steaming was added (point 3017).

FIG. 9 shows the geometric SCT index values obtained when a shoe presswas used at different tilts at a line load 1400 kN/m.

FIG. 10 shows the air resistance measured according to the Gurley method(ISO 5635-5) of the paper obtained when a shoe press was used atdifferent line loads (306-1500 kN/m, resulting in different total pressimpulses) at a constant tilt (1.3). It also shows the Gurley airresistance obtained at a line load of 1500 kN/m when steaming was added(point 3017).

FIG. 11 shows the CCT index values obtained when a shoe press was usedat different line loads (306-1500 kN/m, resulting in different totalpress impulses) at a constant tilt (1.3). It also shows the CCT indexobtained at a line load of 1500 kN/m when steaming was added (point3017). The CCT index is measured in CD.

DETAILED DESCRIPTION

As a first aspect of the present disclosure, there is provided a methodof producing a containerboard.

The containerboard is intended for use as fluting (i.e. corrugatedmedium) in corrugated board. Corrugated board comprises at least onelayer of liner, which is non-corrugated, and at least one layer offluting. In normal production of corrugated board, containerboard iscorrugated and then glued to linerboard. For example, corrugated boardmay consist of a layer of fluting sandwiched between two layers ofliner.

The method comprises a step of pressing a web formed from a pulpcomprising NSSC pulp. The web is generally formed in a wire section, asconventional in the field.

In the head box (i.e. the camber from which the pulp is caused to flowonto the wire of the wire section), the pulp of the present disclosuremay for example have a Schopper Riegler (° SR) (ISO 5267-1) value of20-25 and a Water Retention Value (WRV) (ISO 23714:2007) of 1.7-2.1.After the NSSC pulping process, the SR value may for example be 13-19and the WRV may for example be 1.3-1.7. This means that the pulp of thepresent disclosure may be subjected to refining, such as LC refining,between the NSSC pulping process and the head box.

For example, at least 50% (dry weight) of the pulp of the presentdisclosure may be NSSC pulp. In other examples, at least 55%, 60%, 65%,70%, 75%, 80%, 85%, 90% or 95% (dry weight) of the pulp is NSSC pulp.The part of the pulp not being NSSC pulp may for example compriserecycled fibers. For example, the pulp of the present disclosure mayconsist essentially of NSSC pulp or a mixture of NSSC pulp and recycledfibers. “Recycled fibers” refers to fiber material that has previouslybeen incorporated in some paper or board product. Alternatively or as acomplement, the part of the pulp not being NSSC pulp may for examplecomprise reject pulp. For example, the pulp of the present disclosuremay consist essentially of NSSC pulp and reject pulp. “Reject pulp”refers to pulp prepared by refining the screen reject from anotherprocess.

“NSSC pulp” is obtained from “NSSC pulping”, which in turn is defined inthe background section. The NSSC pulp of the present disclosure may forexample be sodium-based NSSC pulp, which means that the cooking liquorof the NSSC cook comprised Na₂SO₃.

The pressing of the first aspect is carried out in an extended nippress, such as a shoe press. Shoe presses are marketed by severalsuppliers to the pulp and paper industry, such as Voith, Valmet andAndritz. An extended nip press is arranged in the press section of apapermaking machine. Downstream of the press section, the drying sectionis arranged.

Extended nip presses are different from conventional roll presses inthat a longer nip is obtained.

In the method of the first aspect, the line load in the extended nippress is above 1200 kN/m. The “line load” refers to the applied forcedivided by the width of the nip. For example, the line load may be above1300 kN/m, such as above 1400 kN/m, such as at least 1500 kN/m.

Shoe presses are usually not designed for such high line loads, but whenthe inventor specially requested that the line load should be as high aspossible and at least 1500 kN/m, Voith offered a shoe press dimensionedfor 1700 kN/m.

The press pulse in a nip of a press is obtained by dividing the lineload by the machine speed. The total press pulse of a press section isobtained by summing the press pulses of the nips used in the presssection.

The web of the first aspect may for example be subjected to a pressimpulse of at least 102 kPa*s, such as at least 110 kPa*s, such as atleast 115 kPa*s, such as at least 120 kPa*s in the extended nip press.Such press pulses are obtainable in a shoe press with a high line load(see e.g. table 1, below).

The web of the first aspect may for example be subjected to a totalpress impulse of at least 122 kPa*s, such as at least 130 kPa*s, such asat least 135 kPa*s, such as at least 140 kPa*s in the press section.Such press pulses are obtainable in a press section comprising a shoepress used at a high line load (see e.g. table 1, below).

Further, the web of the first aspect may for example be subjected topeak nip pressure of at least 60 bar, such as at least 70 bar, such asat least 80 bar, such as at least 90 bar in the extended nip press, e.g.in the shoe press. As shown in FIGS. 3-4 , such peak nip pressures areobtainable in a shoe press with a high line load. To further increasethe peak pressure, the tilt of the shoe may be increased, as shown inFIG. 4 . The “peak nip pressure” is sometimes referred to as thespecific pressure.

A particular benefit of using a shoe press at a high line load is that acombination of a high peak nip pressure with a high press impulse isobtainable. Such a combination is particularly beneficial in theproduction of containerboard having high compressive strength from NSSCpulp.

The inventor has found that the compressive strength of the producedcontainerboard is substantially increased if the temperature of the webis increased before and/or in the extended nip press. For example, theweb in the extended nip press may be at least 45° C., such as at least50° C., such as at least 55° C., such as at least 60° C., such as atleast 65° C. The temperature may for example be measured with an IRthermometer, such as a hand-held IR thermometer (“IR pistol”). Toincrease the temperature of the web, steam may be applied to it rightbefore and/or in the shoe press. A steam box may for example be arrangedright before the nip of the extended nip press, either below or abovethe web.

The nip length in the extended nip press may for example be at least 150mm, such as at least 200 mm, such as at least 230 mm. In conventionalroll presses, such nip lengths cannot be obtained.

In an embodiment of the first aspect, the web of the first aspect isfurther pressed in a second extended nip press, such as a second shoepress. The conditions in the second extended nip press may be as in the(first) extended nip press discussed above.

The press section employed in the first aspect may also comprise one ormore roll presses (that are not shoe presses). Alternatively, the presssection may consist exclusively of one or more shoe presses. Asconventional in the field, the press section of the first aspect isgenerally followed by a drying section.

As shown in FIGS. 5 and 6 , the pressing according to the first aspectincreases the density of the resulting containerboard and the increaseddensities results in increased compressive strength values. The density(SCAN-P 88:01) of the containerboard of the first aspect may for examplebe above 725 kg/m³, such as at least 740 kg/m³, such as at least 750kg/m³, such as at least 760 kg/m³.

The compressive strength in the machine direction (MD) and the crossdirection (CD) of the containerboard may be measured using a short-spancompressive tester (SCT). The SCT compressive strength (N/m) may bemeasured according to ISO 9895. To calculate the compressive strengthindex, the compressive strength (N/m) is divided by the grammage (g/m²).The unit of the SCT index is thus Nm/g. The grammage of thecontainerboard may for example be 100-200 g/m², such as 100-190 g/m²,such as 110-180 g/m².

The geometric SCT index is calculated as the square root of the productof the SCT index in MD and CD:geometric SCT index=√(SCT index(MD)*SCT index(CD)).

The geometric SCT index of the containerboard of the first aspect mayfor example be above 37 Nm/g, such as at least 38 Nm/g, such as at least39 Nm/g, such as least 40 Nm/g, such as at least 41 Nm/g, such as atleast 42 Nm/g, such as at least 43 Nm/g, such as at least 44 Nm/g.

The compressive strength is considered to be more important in CD thanin MD. The SCT index in CD of the containerboard of the first aspect mayfor example be above 28 Nm/g, such as at least 29 Nm/g.

As second aspect of the present disclosure, there is provided acorrugated board comprising a liner and a fluting. The fluting is formedfrom a pulp comprising NSSC pulp. Various examples of such a pulp aregiven above in connection with the first aspect.

The containerboard used to form the fluting of the second aspect may forexample be obtained using the method of the first aspect.

The density (SCAN-P 88:01) of the fluting of the corrugated board of thesecond aspect is above 725 kg/m³. Higher densities are generallyassociated with higher compressive strengths. According, the density ofthe fluting is preferably at least 740 kg/m³, such as at least 750kg/m³, such as at least 760 kg/m³.

The geometric SCT index (ISO 9895) of the fluting of the second aspectmay for example be above 37 Nm/g. It is preferably at least 38 Nm/g,such as at least 39 Nm/g, such as at least 40 Nm/g, such as at least 41Nm/g, such as at least 42 Nm/g, such as at least 43 Nm/g, such as atleast 44 Nm/g.

As mentioned above, the compressive strength is considered to be moreimportant in CD than in MD. The SCT index in CD of the fluting of thesecond aspect may for example be above 28 Nm/g, such as at least 29Nm/g.

The CCT value may also be used to quantify the compressive strength. Inthe CCT measurement according to SCAN P-42, the sample is corrugated andthe compressive strength is then measured in CD. To obtain the CCTindex, the CCT value is divided by the grammage. The CCT index of thefluting of the second aspect may for example be at least 25 Nm/g, suchas at least at least 26 Nm/g, such as at least 27 Nm/g. Thecontainerboard of the first aspect may also have such a CCT indexmeasured according to SCAN P-42.

The grammage (ISO 536) of the fluting may for example be 100-240 g/m²,such as 100-200 g/m², such as 100-190 g/m², such as 110-180 g/m².

The Gurley air resistance (ISO 5636-5) of the fluting may for example beat least 150 s, such as at least 200 s.

There is also provide a three-dimensional article, such as a box or traycomprising walls composed of the corrugated board according to thesecond aspect. Such a box or tray may for example be suitable for fruitor vegetables.

EXAMPLES

Pilot trials were carried out at Packaging Greenhouse (Karlstad,Sweden). For the pilot trials, refined NSSC pulp was taken from themachine chest on paper machine 6 (PM6) in Gruvön paper mill (Grums,Sweden).

FIG. 1 shows a schematic drawing of the wire section 10 used in thepilot trials. A head box 11 is arranged upstream the wire section 10. Apress section 12 is arranged downstream the wire section 10. Thetemperature of various points in the wire section, as measured by an IRpistol, when a steam box 13 is employed is shown in the figure. Thesteam box 13 is arranged such that the temperature of the web can beincreased from 50 to 70° C. shortly before the press section. However,the temperature falls below 70° C. in the press section, as explainedbelow.

FIG. 2 shows a schematic drawing of the press section used in the pilottrials. After the pick-up roll 21, a double-felted jumbo press (firstpress) 22 is arranged followed by a double-felted shoe press (secondpress) 23. After the second press nip 24, the web is transported througha third press (not shown). In trial 3012 (see below), which did notinvolve steaming, the temperature measured with an IR pistol wasapproximately 47° C. before an after the couch 25, 44° C. after thefirst press 22, 40° C. after the second press 23 and 38° C. on the reel26. In trial 3017 (see below), which involved steaming, the temperaturewas instead approximately 52° C. before an after the couch 25, 53° C.after the first press 22, 49° C. after the second press 23 and 47° C. onthe reel 26.

Tables 1 and 2 below show the different pilot trials. The machine speed(wire) was 730 m/min (which is higher than the machine speed on PM6) andthe target grammage was 140 g/m². The vertical slice lip was 16.1 mm.The samples from the pilot trials of tables 1 and 2 were dried off-linein a one-cylinder dryer.

A first reference trial (3001) was carried out before trials with higherline loads and a second reference trial (3012) was carried out after thetrials with higher line loads. In the reference trials, the same pressimpulse as obtained on PM 6 today was used to simulate the processconditions on PM6.

TABLE 1 Trials with varying line loads. The tilt was 1.3 in all trials.The steam box was turned off in all trials except trial 3017. Firstpress Second/shoe press Total Line Press Line Press press ResultingTrial load, impulse load, impulse impulse density # (kN/m) (kPa*s)(kN/m) (kPa*s) (kPa*s) (kg/m³) 3001 106 8.7 306 25.2 33.9 726 (ref) 3002250 20.5 500 41.1 61.6 802 3003 250 20.5 750 61.6 82.2 815 3004 250 20.51000 82.2 102.7 834 3005 250 20.5 1250 102.7 123.3 841 3006 250 20.51400 115.1 135.6 854 3007 250 20.5 1500 123.3 143.8 859 3017 250 20.51500 123.3 143.8 886 3012 106 8.7 306 25.2 33.9 771 (ref)

Table 1 shows that the density was increased by 9% compared to the mostreliable reference point (3012) by increasing the line load in the shoepress to 1250 kN/m. When the line load in the shoe press was increasedto 1500 kN/m, the density was increased by 11% compared to the mostreliable reference point. When the line load was 1500 kN/m and steam wasadded, the density was increased by 15% compared to the most reliablereference point.

In commercial production using a full-scale drying section (such as theproduction on PM6), the obtained densities are generally lower than inthe pilot trials, wherein a one-cylinder off-line dryer was used. It ishowever expected that the relative increase in density will be about thesame in the commercial production as in the pilot trials when a shoepress is used at a high line load. The density of the containerboardproduced on PM6 has been about 670 kg/m³. It is thus expected that thecommercial containerboard will have a density of at least 725 kg/m³ whena shoe press is used at a line load of at least 1200 kN/m.

TABLE 2 Trials with varying tilt on shoe press Line load, Line load,first press second/shoe press Trial # (kN/m) (kN/m) Tilt Steam box 3008250 1400 1.1 Off 3006 250 1400 1.3 Off 3009 250 1400 1.5 Off 3010 2501400 1.7 Off 3011 250 1400 1.9 Off

FIG. 3 shows nip pressure profiles for the second press (the shoe press)at different line loads and a tilt of 1.3, which is a default value. Inthe reference trial (3001), the maximum nip pressure was below 20 bar.When a line load of 750 kN/m (trial 3003) was used, the maximum nippressure was about 40 bar. When a line load of 1250 kN/m and 1500 kN/mwas used, the maximum nip pressure was about 65 and 75 bar,respectively.

FIG. 4 shows that the maximum nip pressure can be increased byincreasing the tilt. At a line load of 1400 kN/m, the maximum nippressure was above 100 bar for a tilt of 1.9.

FIG. 5 shows that the density increases with an increasing tilt.

FIG. 6 shows that the density increases with an increasing press impulse(kPa*s). In turn, the press impulse increased with an increasing lineload. The second reference trial (3012) resulted in a higher densitythan the first reference trial (3001). The reason for this differencemay be attributed to startup imbalances, such as insufficient finesbalance and/or other effects related to, for example, temperatures,performance of press felts etc. The second reference trial (3012) istherefore considered to give a more representative value.

FIG. 6 further shows that steaming (trial 3017) gives an extra increasein density.

Compressive strength is considered to be the most important property fora corrugated medium. FIG. 7 shows the geometric SCT index obtained atthe different press impulses. In general, the geometric SCT indexincreases with an increasing press pulse. The press pulses generated bythe line loads 1400 and 1500 kN/m, i.e. trials 3006, 3007 and 3017,appear to have a particular influence on the compressive strength. FIG.7 further shows that steaming (trial 3017) gives a significant extraincrease of the geometric SCT index value.

The compressive strength in the CD is particularly important. FIG. 8shows that the SCT index in CD increases with an increasing press pulse.

FIG. 9 shows that not only the density, but also the geometric SCTindex, increases with an increasing tilt. The geometric SCT index wasthus increased when the maximum nip pressure increased.

The densification of the containerboard may also be quantified bymeasuring the air resistance according to the Gurley test. FIG. 10 showsa significant increase in the air resistance when the press impulse wasincreased.

FIG. 11 shows the CCT index at the different press impulses. The presspulses generated at a line load of at least woo kN/m, i.e. trials 3004,3005, 3006, 3007 and 3017, resulted in CCT index values above 25 Nm/g.Line loads below 1000 kN/m resulted in CCT index values below 25 Nm/g.FIG. 11 further shows that steaming (trial 3017) gives a significantextra increase of the CCT index value.

The invention claimed is:
 1. A method of producing a containerboard,comprising the step of pressing a web formed from a pulp comprisingNeutral Sulfite Semi Chemical (NSSC) pulp in a shoe press, wherein theline load in the shoe press is above 1200 kN/m, the web is subjected topeak nip pressure of at least 90 bar in the shoe press and at least 70%by dry weight of the pulp is NSSC pulp, the web is subjected to a pressimpulse of at least 102 kPa*s in the shoe press, and the geometric SCTindex according to ISO 9895 of the containerboard is above 37 Nm/g. 2.The method of claim 1, wherein the temperature of the web in the shoepress is at least 45° C.
 3. The method of claim 1, wherein steam isapplied before or in the shoe press to heat the web.
 4. The method ofclaim 1, wherein the nip length in the shoe press is at least 150 mm. 5.The method of claim 1, wherein the pulp comprises recycled fibers and/orreject pulp in addition to the NSSC pulp.
 6. The method of claim 1,wherein the density according to SCAN-P 88:01 of the containerboard isabove 725 kg/m³.
 7. The method of claim 1, wherein the line load in theshoe press is above 1300 kN/m.
 8. The method of claim 1, wherein theline load in the shoe press is above 1400 kN/m.
 9. The method of claim1, wherein the web is subjected to a press impulse of at least 110 kPa*sin the shoe press.
 10. The method of claim 1, wherein the web issubjected to a press impulse of at least 120 kPa*s in the shoe press.11. The method of claim 1, wherein the temperature of the web in theshoe press is at least 55° C.
 12. The method of claim 1, wherein thetemperature of the web in the shoe press is at least 65° C.
 13. Themethod of claim 1, wherein the nip length in the shoe press is at least230 mm.
 14. The method of claim 1, wherein the geometric SCT indexaccording to ISO 9895 of the containerboard is at least 38 Nm/g.
 15. Themethod of claim 1, wherein at least 75% by dry weight of the pulp isNSSC pulp.
 16. The method of claim 1, wherein at least 80% by dry weightof the pulp is NSSC pulp.
 17. The method of claim 1, wherein thegrammage of the containerboard is 100-200 g/m².